The present invention relates to an apparatus for measuring cross-sectional distribution of refractive index of an optical waveguide, which is used for optical communication.
As a method for measuring cross-sectional distribution of refractive index of an optical waveguide, there is the refracted near field method (RNF method). This RNF method is non destructive and provides high measurement accuracy and high resolution and is considered as the most excellent method for measuring cross-sectional distribution of refractive index of an optical waveguide.
According to this RNF method, an optical waveguide substrate 1 comprises an optical waveguide portion 3 on one side of the substrate portion 2, and this is immersed in a liquid 9 having refractive index of n.sub.L, which is closer to the refractive index n(r) of the optical waveguide portion 3 as shown in FIG. 15. Under this condition, laser beam converged by an objective lens 8 on an end surface of said optical waveguide portion 3 is irradiated at an incident angle .theta., and the light leaking through the optical waveguide portion 3 is detected, and the refractive index of the optical waveguide portion 3 is measured.
When the refractive index of the optical waveguide portion 3 at the point where laser beam enters is supposed to be n(r), and that the refractive index of the air or the liquid, of incident side on optical waveguide portion 3 is n.sub.0, the exit angle .beta. relative to the incident angle .theta. is given simply by the following equation (1) in accordance with Snell's law: EQU n.sup.2 (r)=n.sub.o.sup.2 sin.sup.2 .theta.+n.sub.L.sup.2 cos.sup.2 .beta.(1)
Therefore, when the incident point of laser beam is scanned toward the direction of the thickness of the optical waveguide portion 3, or the direction of crossing at right angles with the above direction, the exit angle .beta. changes according to the refractive index n(r) at each point. Specifically, the exit angle .beta. is decreased at the portion having higher refractive index, and it is increased at the portion having lower refractive index.
Accordingly, by judging the condition of the leaking light, the refractive index n(r) of the optical waveguide portion 3 can be obtained.
The apparatus for measuring cross-sectional distribution of refractive index of optical waveguide according to RNF method is based on the above principle.
Continuing the explanation in connection with FIG. 15, a detector 5 for receiving the light 4 leaking through said optical waveguide portion 3 is provided on lateral side of the optical waveguide portion 3. Also, a shielding plate 6 of semi-circular disk is provided for shielding a part of the leaking light 4 closer to the center. Said detector 5 receives the leaking light 4 in semi-doughnut shape, lacking a part of said detector 5 closer to the center. The light receiving quantity is given by the following equation (2), where the angle of the leaking light to the light receiving point on the outermost side is .theta..sub.max and the angle corresponding to the inner light receiving point shielded by said shielding plate 6 is .theta..sub.min. ##EQU1##
In the above equation, I(.theta.) represents intensity distribution based on the angle dependency of the incident light. By sufficiently increasing the light receiving surface of the detector 5, the leaking light 4 is prevented from going out of the light receiving surface. Therefore, .theta..sub.max in the above equation (2) is determined by the numerical aperture (NA) and is given by the following equation (3): EQU n.sub.0 sin .theta..sub.max =NA (3)
The above exit angle .beta..sub.max is changed according to the refracting powder of the optical waveguide portion 3, that is, the light receiving point on the outermost side of the leaking light is moved, whereas it is primarily determined by the point where the detection light enters the optical waveguide portion 3 and also by the position of the edge of said shielding plate 6 and is not influenced by the refractive index of the optical waveguide portion 3.
Further, the incident angle .theta..sub.min corresponding to the above exit angle .beta..sub.min is obtained by the equation (4), which is a variant of the above equation (1). EQU n.sub.0.sup.2 sin.sup.2 .theta..sub.min =n.sup.2 (r)-n.sub.L.sup.2 cos.sup.2 .beta..sub.min ( 4)
The incident angle .theta..sub.min is an important factor to determine the refractive index of optical waveguide portion 3. Specifically, the light quantity obtained by the above equation (2) is changed according to the refractive index.
When it is supposed that the light receiving quantity at an arbitrary point in the direction of the thickness of the above optical waveguide portion 3, or the direction of crossing at right angles with the above direction is P(n(r)), this light receiving quantity P(n(r)) is given by the following equation (5): ##EQU2##
Next, if the angle dependency I(.theta.) of the incident light intensity has Lambert distribution [I(.theta.)=I.sub.0 cos .theta.], the equation (6) is obtained from the equation (5), and .DELTA.n (r) can be obtained when the laser spot position is scanned in the direction of the thickness of the optical waveguide portion, or the direction of crossing at right angles with the above direction and the change of light quantity .DELTA.P is measured. EQU .DELTA.P=a.multidot..DELTA.n(r) (6)
Here, the proportional constant a is determined by the refractive index n.sub.L, which is already known.
Normally, laser beam is used as the light source. In this case, incident light intensity distribution I(.theta.) is Gauss distribution rather than Lambert distribution, and the light quantity change and the change of refractive index are not so simple as in the equation (6). Through the correction by calculation, .DELTA.n (r) can be obtained.
In a conventional type apparatus for measuring cross-sectional distribution of refractive index of an optical waveguide, the optical waveguide substrate is immersed in a liquid which has the refractive index closer to, or more preferably, higher than that of the optical waveguide portion in order to prevent total reflection of light within the optical waveguide portion and to irradiate the light entering the optical waveguide portion to outside the optical waveguide portion. For this reason, the conventional type apparatus requires the liquid for immersion.
In case the material of optical waveguide portion is glass, the refractive index is about 1.5, and it is relatively easy to select the liquid for immersion. However, in case the optical waveguide portion is formed by thermal diffusion of Ti on single crystal substrate of LiNbO.sub.3 or LiTaO.sub.3, the refractive power of the substrate is 2.0 or more in many cases.
For such optical waveguide substrate, the liquid for immersion having similar refractive index is needed, whereas the liquid having the refractive index of 2.0 or more is harmful to human body. Also, even when the refractive index is low, there may be no liquid for immersion suitable for the substrate, depending upon the material of the substrate. For this reason, it is diffifult and dangerous to measure, or it is impossible to measure in some cases.
The object of the present invention is to make it possible to measure the distribution of refractive index of optical waveguide even when the liquid for immersion is not provided.